Plasma polymer membranes, or those prepared by plasma polymerization, show great potential for commercial fluid separations because they can be made highly selective for the permeation of one species over another in a mixture. This high selectively is due to their highly crosslinked structure in which intermolecular spacings are rigidly maintained--thus creating holes through the polymer network of reasonably uniform size. Permeating molecules pass through the holes in the polymer network and the rate of permeation is greatly influenced by the size of the permeating molecules. This so-called "sieving mechanism" for molecular permeation is believed to be responsible for the very high selectivities that have been observed for some plasma-polymerized membranes.
Plasma polymer membranes are also potentially useful as barrier films to provide a seal against fluids such as water or air. In such cases, the polymer is prepared in such a way to exhibit a very dense and highly crosslinked impermeable structure. However, two major drawbacks have kept plasma polymer membranes from wide commercial application: (1) they tend to be brittle and highly prone to fracturing; and (2) they generally have low permeabilities and thus must be made exceedingly thin to obtain practical fluxes in separation applications.
The brittleness of plasma polymer membranes is due to their highly crosslinked structure and due to the plasma polymerization mechanism itself. This subject is discussed in detail by Yasuda in Plasma Polymerization Academic Press, New York, N.Y., 1985, and by Yasuda et al., in 46 J. Membrane Sci. 1 (1989). During the plasma polymerization process internal stresses build within the polymer film and increase with increasing film thickness. At some point, the internal stress within the film exceeds the cohesive strength of the plasma polymer and cracking occurs; or the adhesive strength between the plasma polymer film and the adjacent polymer film layer or a rigid substrate is exceeded and delamination occurs. The internal stress of plasma polymer films and their tendency to form cracks and other defects can be reduced by such factors as reducing the film thickness, the proper choice of monomer, and by selection of plasma polymerization conditions.
In the fabrication and use of practical separation devices using plasma polymer membranes, four principal modes of failure have been observed: (1) as prepared, the membranes contain defects or form cracks which result in the loss of selectivity or barrier properties; (2) the membranes are mechanically weak and are breached during handling or when subjected to pressure; (3) during their incorporation into modules such as in the preparation of spiral-wound modules which involves bending the membrane sheets around radii as small as 1 cm, delamination or cracking occurs; and (4) during operation the membranes flex due to the applied operating pressures of 100 psi or more and the membranes crack due to the repeated flexure.
Various solutions to the problems of brittle, physically weak, and defective plasma polymer membranes have been proposed. For mechanical strength, composite plasma polymer membranes are generally prepared by depositing the highly selective but physically weak plasma polymer film on a strong and highly permeable substrate. It has been recognized that plasma polymerization of a brittle and highly selective film directly on a porous substrate is undesirable because the films must be made relatively thick (approximately 5 times the pore diameter of the porous substrate) to completely bridge the surface pores. Such composite membranes exhibit unacceptably low flux and are prone to defects due to the thickness of the plasma polymer layer. Stancell and Spencer originally proposed a solution to this problem in 16 J. Appl. Polym. Sci. 1505 (1972) by preparing a thin plasma polymer layer on either side of dense, but highly permeable, conventional polymer films such as poly(phenylene oxide) or silicone-carbonate copolymer. Three-layer plasma polymer composite membranes comprising a microporous support membrane, a first dense and permeable layer such as silicone rubber or plasma-polymerized siloxane monomer, and a second thin and more selective plasma polymer layer are described in U.S. Pat. Nos. 4,483,901, 4,533,369, 4,696,686, and 4,976,856. A four-layer composite membrane is disclosed in U.S. Pat. No. 4,581,043, which is essentially the same as the three-layer composites just mentioned, except that an optional fourth permeable layer may be applied over the plasma polymer layer to seal defects therein and to provide protection from mechanical damage. U.S. Pat. No. 4,410,338 and Kramer et al., in 46 J. Memb. Sci. 1 (1989), both disclose three-layer composite gas separation membranes comprising two thin plasma polymer layers on a microporous support membrane, one of the layers being highly permeable, while the other is highly selective. Although the '338 patent discloses the fabrication of such composite membranes into modules, the highest oxygen-to-nitrogen selectivity reported is only 4.2. Buck et al., in 2 Br. Polym. J. 238 (1970), discloses a multi-layer plasma polymer RO membranes consisting of a porous support membrane, two consecutive coats of polyhexamethyldisiloxane and up to 15 consecutive coats of polyvinylene carbonate. However, flux through the membranes decreased dramatically with increasing layers, and there was no suggestion of alternating permeable and selective plasma polymer layers.
Another deficiency of previously disclosed multi-layer plasma polymer membranes is that they frequently do not possess the high intrinsic selectivity of the selective plasma polymer layer. For example, Kawakami et al., in 19 J. Membrane Sci. 249 (1984), disclose membranes with plasma-polymerized layers over natural rubber and silicone rubber that exhibit calculated oxygen-to-nitrogen selectivities of up to 15.8 for the plasma layer, yet the actual measured selectivities of the composite membranes were only in the range of 3.1 to 5.8. It therefore appears that the previously reported multi-layer plasma polymer membranes either contained minor defects or were prepared in such a manner that the plasma polymer layer does not provide the majority of the overall membrane resistance and therefore does not substantially characterize the membrane's overall selectivity.
There therefore exists a need for improved multi-layer plasma polymer membranes that exhibit the high intrinsic selectivity of the plasma polymer layer and retain this high selectivity after bending and repeated pressurization and depressurization conditions that are common in the fabrication and use of commercial devices. This need is met by the present invention, which is summarized and described in detail below.